The present invention concerns telecommunications.
It is more particularly concerned with an asynchronous transfer mode digital telecommunication network terminal equipment synchronization device.
These networks are well known and will not be described again here. Suffice to say that in a network of this kind the data is structured into fixed length packets or cells each comprising one part reserved for data to be transmitted by the network and a header which includes parameters needed to route the cell through the network and that the cells are routed through the network utilizing the virtual circuit technique, a hybrid of circuit switching and packet switching well suited to transmission through a telecommunication network at various bit rates, including very high bit rates and bit rates that vary irregularly with time.
The transfer time-delays introduced by these networks are variable in time for the same call depending on the instantaneous load at the network nodes.
For some applications including telephony where it is necessary to achieve some degree of synchronism between terminals connected by the network it is then necessary to synchronize the terminals in order to achieve in each terminal correct recovery of data received from the network.
A known way of synchronizing a terminal is to retrieve the transmit clock at the receiving end simply by filtering the transfer jitter introduced by the network.
For example, the transfer jitter can be filtered by continuously varying the frequency of a local clock signal according to measured differences between how much of a buffer is filled by the received data and how full this buffer is expected to be, this corresponding to the current frequency state of this signal.
The drawback of this method is that it is not totally effective in the presence of relatively severe constraints on obtaining the filtered clock signal because, failing improvements which would inevitably render it more complex to implement, it cannot react smoothly and smooth action is essential in some applications.
One such application, for example, involves setting up a call between two terminals one of which is connected to an asynchronous transfer mode network such as the broadband ISDN and the other of which is connected to a synchronous transfer mode digital telecommunication network such as the narrowband ISDN, each of these networks having access points to the other.
An application of this kind therefore raises the problem of the compatibility of the two networks, which are by definition based on different principles, in particular using different interfaces with the terminals connected to them.
The term "interface" encompasses all the concepts relating to the nature of the signals exchanged on the medium connecting the terminal in question to the network in question and the procedures for setting up, maintaining and clearing down a call via the network concerned. The interfaces for each type of network are covered by standards.
The standard defining the So interface between a subscriber and the asynchronous transfer mode ISDN requires each call set-up phase to include, prior to the signalling phase relating to the call, a phase for synchronizing the terminals in question to the network reference clock by sending specific signals to the terminals via the network from which they can recover the reference clock. The signalling and data sent to this interface are structured into frames comprising locations with a fixed assignment either to signalling or to data.
In the case considered here of connecting a terminal to this network via an asynchronous transfer mode network, the standard defining the So interface has to be complied with and the terminal is associated with an adapter and the reference clock signal applied to the terminal outside the synchronized phase is produced by a local clock internal to the adapter.
The clock signal from this local clock enables exchange of signalling and therefore call set-up. Unlike data, signalling is generated sporadically, so the problem of synchronous recovery of these signals does not arise.
However, as soon as the exchange of call data begins it is necessary to change from this "local"synchronization to a "distant" synchronization obtained by recovering the send clock from the received data by filtering the jitter affecting the received data, according to the principle mentioned above.
Because of the difference between the local and distant clock frequencies, to prevent excessively high bit rates in the recovery of the received data preserving local synchronization would make it necessary to provide a buffer of excessively large capacity to store this data prior to its recovery.
Also, in this type of application the standard defining the So interface is relatively constricting with regard to the maximum permitted jitter for a clock signal of this kind because the limit that must not be exceeded is in the order of 300 nanoseconds for the highest frequency components of the jitter (above 50 Hz in this instance) whereas the transfer jitter on an asynchronous transfer mode network can exceed 100 microseconds.
It follows that this change of synchronization regime must be carried out smoothly and meet very demanding requirements as to the quality of filtering.
This requirement for smooth changeover from one synchronization mode to another is found in this type of application in the case of two terminals on the same subscriber premises connected to the network by a shared adapter with two separate communication channels (B1 and B2) respectively assigned to the two terminals, as provided for in the standard defining the So interface. This applies when a call involving one of these terminals and using one of these channels is cleared down while another call involving the other terminal and using the other channel continues and distant synchronization was obtained for the former call (assuming this was the first to be set up).
This is also the case if two calls are in progress simultaneously, the device for achieving distant synchronization having to be able, when the second call is set up, to use the distant synchronization achieved for the call set up first without this disturbing, at call set-up time, the recovery of data relating to the second call.